8 minutes of writing at the start of class to help students bring to mind their homework reading or lab work or previous lectures.
8 minutes in mid class when things go dead--or to get students to think about an important question that has come up.
8 minutes at the end of class or lecture to get them to think about what's been discussed.
5 minutes at the end of class to write to us about what they learned that day: what was the main idea for them, what was going on for them during that class. Not only will this help them integrate and internalize the course material; it helps our teaching by showing us what's getting through and what isn't.
Source: http://www.ntlf.com/html/lib/bib/writing.htm